The correct answer is B) Curtis stage, impulse turbine. The astern element of a main propulsion turbine is typically designed as a Curtis stage, impulse turbine. In an impulse turbine, the steam expands through a nozzle, creating high-velocity steam jets that impinge on the blades, causing them to rotate. This design is well-suited for the astern (reverse) element of a propulsion turbine, as it can efficiently generate the necessary torque for reversing the direction of the propeller. The other options are incorrect because: A) Parsons stage turbines are typically used for the ahead (forward) element, not the astern element; C) multiple entry, helical flow turbines are a different design not commonly used for the astern element; and D) single entry, double flow turbines are also a different design not typically used for the astern element.